The present invention relates to a processing device for changing magnification of image data which is applicagle to a digital copier, a facsimile apparatus and other image processing apparatuses.
Referring to FIG. 1 of the drawings, a prior art image reader is shown which resembles in configuration an upper part of a copier. The image reader, generally 1, includes a glass platen 2 to be loaded with a document, and a presser plate 3 adapted to press the document against the glass platen 2. An operating board 4 is provided with several kinds of keys such as a read start button and a density selection key, and several kinds of displays for displaying conditions selected as well as operating conditions of the image reader 1. As an operator depresses the start button, the image reader 1 will begin to read a document on the glass platen 2 to produce an image signal.
FIGS. 2 and 3 show typical examples of the construction of the image reader 1, particularly its optical arrangement. Specifically, FIGS. 2 and 3 show, respectively, an optical arrangement of the type using a direct contact type image sensor and a one of the type using a reduction type image sensor. In another construction known in the art, a document is moved relative to a stationary optical system. In the arrangement shown in FIG. 2, the optical system serves as a 1-magnification optical system. While a luminescent lamp 5 illuminates a document which is laid on the glass platen 2, a reflection 8 from the document is incident to an image sensor 7 by way of a lens 6. The image sensor 7 has a width which is equal to or greater than that of a document (in a direction perpendicular to the sheet surface of FIG. 2, i.e., a main scanning direction X), so that it is capable of reading one line of image data at a time in the widthwise direction. With respect to one line, the sampling number and the sampling pitch Px are determined by the number of pixels of the image sensor 7. On reading one line of data, a carriage 9 on which the lamp 5, lens 6 and image sensor 7 are mounted is driven in a direction indicated by an arrow (subscanning direction Y) so as to read the next line. Alternatively, the carriage 9 may be continuously driven in the subscanning direction Y, as also known in the art. While the interline pitch Py is dependent on the velocity of the carriage 9, the charge storage time of the sensor 7 and other factors, it is usually the same as the sampling pitch Px mentioned above.
In the arrangement shown in FIG. 3, a lens 14 reduces the width of an optical image which is representative of a document to match it to the size of the image sensor 7. The optical system of FIG. 3 is shown to use three mirrors, but two or five mirrors may be used as desired. As regards the readout in the main scanning direction X, the system of FIG. 3 is operated with the same principle as that of FIG. 2. As for the readout in the subscanning direction Y, a first carriage on which the lamp 5 and a first mirror 11 are mounted and a second carriage on which mirrors 12 and 13 are mounted are driven independently of each other such that the optical path from a document on the glass platen 2 to a lens 14 remains constant in length.
In a prior art variable-magnification system, a reduction ratio is varied in the main scanning direction X by varying the length of the optical path of an optical system and, in the subscanning direction Y, by varying the velocity of a movable body. Such an implementation, however, cannot be adopted when it comes to the system of FIG. 2 which uses a direct contact type image sensor. Even the system of FIG. 3 has a problem that the range of magnifications available is limited for structural reasons, e.g., the magnification cannot be noticeably changed despite substantial shifts of the lens 14 and sensor 7. Another problem with the FIG. 3 system is that the lens 14 and sensor 7 have to be moved and positioned by extremely accurate mechanisms in order to eliminate deformation of images read out.
In the light of the above, the current trend in the imaging art is toward the use of electrical variable magnification in place of the above-described optical variable magnification. The electrical variable magnification principle is such that data with a changed magnification are estimated based on a 1-magnification data read out, thereby producing image data for a particular magnification selected. However, the electrical variable magnification scheme currently proposed cannot fully meet the demand for accurate change of magnification and, if elaborated to accomplish accurate magnification changes, it would require complicated hardware and fail to readily implement so-called zoom type variable magnification which is effected on a 1% basis, and a wide range of magnifications.
The problems discussed above may be reduced by adopting the following sequence of steps: counting a data clock DCLK, which is representative of pixel unit divisions of original image data, to use each count as a position i of image data of a particular magnification, computing an integer Ji and a decimal Ri of an equation 100i/[specified magnification R(%)]=Ji+Ri every time one pulse of the data clock DCLK is produced, i.e., every time the number i is increased, sampling image data at a position x=Ji of the original image data and image data next thereto, computing image data of a particular magnification by using the sampled original image data and the decimal Ri, and determining the resultant data as image data of a particular magnification which is located at the "i" position in terms of the data clock DCLK. Such a procedure allows image data of a particular magnification to be produced in synchronism with the data clock DCLK associated with original image data, whereby the image data of a particular magnification can be printed, transferred, transmitted or otherwise processed by raster scanning which is synchronous to the readout or the transfer of the original image data. Moreover, the magnification R can be selected on a 1% basis and over a wide range.
As described above the sampling position x=Ji of original image data and the deviation Ri between the position i of image data of a particular magnification and the position x are computer every time a pulse of the data clock DCLK appears, i.e., every time the original image data is shifted by one pixel. This brings about a problem that should the number of figures of the deviation Ri be substantial, the computing time would limit the frequency of the data clock DCLK available. Specifically, one period of the data clock DCLK should be sufficiently longer than the sum of the computing time mentioned above and a time necessary for image data of a particular magnification to be produced based on the sampled original image data and deviation Ri. Since the period of the data clock DCLK is dependent on the image reading speed, an increase in the period of the data clock DCLK would slow down the image reading operation as well as image data recording, transfer, transmission and other speeds. In addition, the substantial number of figures of the deviation Ri translates into a substantial number of signal bits which are needed for Ri processing, resulting in complicated signal lines and complicated hardware for computations.
Generally, when an image is read by a scanner, the spatial frequency characteristic of an image which is represented by the data read is changed to deteriorate the image. A countermeasure heretofore adopted against such an occurrence is applying MTF (Modulation Transfer Function) compensation to image data at a predetermined stage of operation. This kind of compensation is such that a coefficient pattern (filter) is set up such as shown in FIG. 4A and, for example, pixel data Oik observed (here, data representative of density) as shown in FIG. 4B is corrected to become data Milk which is equal to Y.Oi.sub.-1 k+V.Oik.sub.-1 +W.Oik.sub.+1 +Z.Oi.sub.+1 k+X.Oik. The compensation coefficients V to Z (filter coefficients) may be exemplified by those shown in FIG. 5B. Since those compensation coefficients have optimum values which correspond to the spatial frequency characteristic (sampling density) of original image data, values corresponding to the original image sampling density of a scanner are selected. Hence, for a new sampling frequency at the time of a change of magnification (sampling density of magnification-changed image data corresponding to original image), the compensation coeffidients have to be changed due to the change in adequate MTF compensation characteristic. Especially, it is difficult to directly apply the compensation coefficients adapted for a 1 magnification (FIG. 5B) to a wide range of magnifications from 50% to 400%.
Should the MTF compensation be effected with a reduced or enlarged magnification and with the same compensation coefficients neglecting the above requisite, the edges of an image would be excessively accentuated by the MTF compensation during enlargement, resulting in oscillation (stripe pattern) of the image.
To eliminate the above occurrence, different compensation coefficients may be used for individual magnifications, as shown in FIGS. 5A to 5D by way of example. However, such a scheme cannot be implemented without assigning different hardware or computing programs to the magnification-changed image data computation which relies on interpolation, and the MTF compensation.
Further, when the magnification is to be changed by one ratio in the main scanning direction and by another ratio in the subscanning direction, a single kind of MTF compensation cannot provide adequate MTF-compensated values because the optimum MTF compensation coefficients depend on the direction.